Aircraft radar cross section is becoming an increasingly important parameter in the design of military aircraft. All aircraft reflect interrogating radar signals. However, it is important that the magnitude of the radar reflectivity, or radar cross section, be minimized to make military aircraft less observable to various types of radars. The radar cross section of an aircraft is typically measured by placing a scale model of the aircraft in an anechoic chamber having radar absorbing walls, interrogating the model with a pulsed radar signal and measuring the magnitude of the radar signal reflected from the model. The model, also known as a test target, is generally supported in the anechoic chamber by either several downwardly extending strings or an upwardly extending pylon. Regardless of whether the test is supported by strings or a pylon, it is critical that the strings or pylon have a lower radar cross section than the test target. Otherwise, the magnitude of the measured radar reflections provides an indication of the radar cross section of the strings or pylon rather than the radar cross section of the test target. At relatively low frequencies, it is generally preferable to support the test target with strings since the radar cross section of strings is generally lower than that of pylons at low frequencies. At higher frequencies, the radar cross section of a pylon is generally significantly less than the radar cross section of strings. Thus, at the relatively high frequency at which many radar systems operate, it is preferable to utilize a pylon to support the test target.
Conventional pylons for supporting a test target in an anechoic chamber are generally in the form of an elongated inclined support or column terminating in a tip on which the test target is mounted. The test target is generally mounted on the tip of the pylon through a rotator to rotate the test target around its yaw axis. The test target is also mounted on an actuator that rotates the test target about its pitch axis. The support is generally tapered from its base to the tip, and it is generally ogival in cross section. Such pylons are manufactured, for example, by Scientific Atlanta Company.
The radar cross section of conventional pylons for supporting test targets is typically -30 dBsm at 2 GHz. At lower frequencies, the radar cross section of these pylons increases rapidly to about -20 dBsm at 1 GHz. The pylons are typically coated with a radar-absorbing material to improve the low frequency performance, but the improvement is generally no more than about 2-5 dB. As frequency increases, the radar cross section of conventional pylons improves to about -50 dBsm at 8 GHz, and radar-absorbing coatings can improve this performance about 15-20 dB.
While the conventional pylon described above has in the past proven satisfactory for most applications, it is becoming more and more inadequate as the radar cross section of new aircraft designs continues to decrease. More specifically, the radar cross section of test targets simulating modern, low observable aircraft is approaching the radar cross section of the pylon so that it is becoming impossible to measure the radar cross section of the test target alone. The principal contributor to the radar cross section of the pylon is the interface between the pylon and test target. At low frequencies, the finite length of the pylon causes diffraction to be generated at the pylon/target interface. As frequency increases, diffraction becomes less important but surface roughness effects limit the radar cross section performance of the pylon. There is thus a need for a low radar cross section pylon that is capable of supporting a large, heavy test target at a variety of angular orientations.